1. Field of the Invention
This invention relates to enclosures for video cameras in surveillance systems, and in particular, to an outdoor enclosure for a video camera in a surveillance system which can withstand a wide range of ambient temperatures and weather conditions without adversely affecting operation of the video camera. This invention also relates to video surveillance systems employing such outdoor video camera enclosures.
2. Description of Related Art
A long standing problem faced by those skilled in the art was to invent a environmentally sealed enclosure for a video surveillance dome camera that can operate over a wide range of ambient temperatures and weather conditions, for example over a temperature range of approximately -40.degree. C. to +50.degree. C., and weather conditions ranging from bright, sunny and hot to rain, snow and frigid temperatures. Further, such enclosures must operate with an input power level limited as a Class 2 power supply of only 80 VA or less, for safety reasons. Moreover, due to the size reduction of new surveillance dome cameras over previous products, it is desirable and necessary to reduce the size of the enclosure as compared to previous products. Such reductions in size greatly limit the size of fans that can be used in such enclosures to approximately 30 mm.sup.2. The standard fans of this dimension are typically limited to 3.5 cubic feet per minute of volume air flow. This is a relatively small airflow that creates a challenge for a design to achieve the required cooling and heating at temperature extremes. In addition to the basic technical challenges, market pressures dictate that new enclosures should be more aesthetically pleasing and cost substantially less than existing designs. Besides being necessary to reduce the size because of the power limitation, it is desirable to improve the appearance by making it smaller, more stylish and less obtrusive. A final but critical challenge is to achieve high reliability, since it is generally difficult to access the surveillance cameras for servicing as they are usually at a high outdoor location.
Prior solutions use fans in various arrangements to blow warm air down over the bubble from many points equally spaced around the circumference of the bubble. This is to provide an even distribution of warm air over the bubble to provide for the melting of frost or ice. Many of these designs blow or draw air through a duct or tube that has a heater inside of it to produce the warm air. These designs require a larger fan than the size noted above, capable of the higher static pressures needed to produce a significant air velocity out of multiple points on a distribution manifold. Alternative designs have the fans mounted in relatively free space so as to draw air over a heater located in close proximity to the fan. This method allows the air that is exiting the fans to immediately return to the fan intake and does not promote air circulation through the entire housing and bubble. This poor circulation decreases the housing's ability to transfer heat out of the dome in warm environments, and to warm the optical surface of the housing in cold environments.
Outdoor video surveillance camera enclosures in accordance with the prior art are sold by Sensormatic Electronics Corporation, Pelco Corporation and Vicon Corporation. The camera mounting assembly utilized herein is described in U.S. Pat. No. 5,649,255--Schieltz, a coinventor herein.